The present invention relates to an image-forming device such as an electrophotographic printer or a copying machine, in particular, an image-forming device having a primary charger for charging a photosensitive belt.
In the image-forming device such as the electrophotographic printer and the copying machine, a charger charges a charged object as a photosensitive body rotating in a direction to a predetermined potential V0 by corona discharge. Charging by corona discharge has a good property of uniformly charging the photosensitive body. However, since a high voltage of about 4 through 6 kv in direct current is used, any amount of ozone is generated at the occurrence of corona, which causes environmental damage. As a countermeasure, Japanese Examined Patent Application No. H3-52058 discloses a contact-type charger which charges a photosensitive body to a desired potential by relatively low voltage. The contact-type charger has a roller that generates an extremely small amount of ozone and contacts the photosensitive body. An alternating voltage is applied to the contact-type charger.
The charger disclosed in Japanese Examined Patent Application No. H3-52058 achieves uniform charging the photosensitive body with the generation of an extremely small amount of ozone (about one-tenth through one-hundredth as compared to the charger using corona discharge). Since the charger disclosed in Japanese Examined Patent Application No. H3-52058 has a high capability of eliminating potential history on the photosensitive body, an eliminating process (an eliminating device) prior to a charging process is not required and thus, the size of the image forming apparatus can be reduced. However, in this charger, size of an alternating voltage source of this charger is lager than that of direct voltage source and vibration noise is generated at a nip area between the charger and the photosensitive body due to an AC electric field therebetween.
A method of charging the photosensitive body by using DC electric field without using the AC electric field is disclosed in Japanese Patent Application Publication No. H6-348112, for example. According to the method described in Japanese Patent Application Publication No. H6-348112, when only DC voltage is applied to a charger, a surface of a photosensitive body tends to be nonuniformly charged. Then, striped unevenness of charging having 2 through 200 mm in length and 0.5 mm or less in width in a direction perpendicular to a moving direction of the surface of the photosensitive body occurs thereon. The striped unevenness of charging on the photosensitive body appears as an image defect such as a black stripe in reverse development method and a white stripe in nonreverse development method (a white stripe occurs in all black or halftone).
Japanese Patent Application Publication No. H10-198132 describes a charging method in which a charger charges a photosensitive body at gaps between a charging roller and the photosensitive body on upstream and downstream sides in a rotational direction of the photosensitive body with respect to a contact position of the charging roller and the photosensitive body. In this charging method, difference in potential between the charging roller and the surface of the photosensitive body is insufficiency, resulting in unstable discharging therebetween. Thus, the surface of the photosensitive body is nonuniformly charged, thereby appearing the above-described black or white stripe on a recording medium
As a solution for the above-described problem, Japanese Patent Application Publication No. H6-348112 discloses a charging method in which the charging potential on an electrophotographic photosensitive body is set within the range of 300 through 650V. Further, in the charging method of Japanese Patent Application Publication No. H10-198132, the charging roller charges the photosensitive body in the gap only on the downstream side, while an erasing device removes potential from the surface of the photosensitive body on the upstream side of the contact position.
According to the charging method in Japanese Patent Application Publication No. H6-348112, a photocarrier on the photosensitive body generated by being exposed to a laser beam of the erasing device may remain on the photosensitive body even after the passage through nip area between the charging roller and the photosensitive body, thereby eliminating the potential charged by the charging roller in the gap on the downstream side. This leads to low charging efficiency and nonuniform charging. Consequently, when a distance between the most downstream point of an irradiation region of the erasing device and the charge starting point of the charger is defined as L(mm), a moving speed of photosensitive body is defined as V(mm/sec) and a life of the photocarrier on the surface of the photosensitive body generated by being exposed to the laser beam of the erasing device is defined as τ (sec), uniform charging on the surface of the photosensitive body is achieved by setting the L/V≧τ.